Create gist from selection - will create a gist using your GitHub credentials (see below). It will either use the current selection, or the whole file, using the filename and the currently active language. The URL of the gist will be copied to your clipboard.

Create private gist from selection - same as above, but the Gist will be private.

To enable Gist support, make sure you've followed the instructions on your account page for adding your GitHub user and API Token to your global Git config. You can
create gists without specifying your credentials, but they won't be associated with your account.